Memories may be categorized into ROM (Read Only Memory) and RAM (Random Access Memory). ROM can keep data without electricity. RAM loses data stored thereinside once power is turned off. ROM can be further classified into mask ROM, EPROM (Erasable Programmable ROM), EEPROM (Electrically Erasable Programmable ROM), and flash memory. As disclosed in a U.S. Pat. No. 5,514,610, high-energy ion is used to write data in mask ROM. Once data has been written into mask ROM, the user cannot erase or rewrite the data but can only read the data. Therefore, mask ROM normally applies to the products that data is not intended to vary in the future, such as the booting programs for computers/embedded devices, font tables, and programs for game machines. Mask ROM has the advantage of the lowest cost per bit.
Mask ROM normally adopts channel transistors as the memory units and turns on/off the memory units through changing the threshold voltage. In mask ROM, word lines (WL) span bit lines (BL) to form an array structure corresponding to every memory unit, whereby binary data “0” and “1” are written or read. The width of BL and WL and the area of memory units directly influence the size of mask ROM.
At present, mask ROM is usually applied to diverse small-quantity products and normally fabricated with the 6-inch process. The 6-inch process generally adopts the 365 nm I-line exposure machine, which can only fabricate 365 nm wide lines at best. Thus, it is hard for mask ROM to reduce the overall area, increase the memory unit density and promote the storage capacity.